My primary research interest is the fluid dynamics of stratified flows. I enjoy revealing their beautiful structures with experiments and strive to understand them using mathematical analysis. Since 2022, I have also been co-leading the UK Fluids Network 'Special Interest Group' on Ocean Turbulence. More recently, I developed an interest in the modelling of decompression sickness in scuba diving using hydrogels to mimic human tissues.
Since "a picture is worth a thousand words, and a video is worth a million" I made a few short (1 to 3 minutes) videos to introduce my research into stratified shear flows, why they're fascinating, and why they're important.
What do I study?
Why do I find it fascinating?
Why is it important? (1/2)
Why is it important? (2/2)
Finding Nessie: an artistic view
Below is a list of my main projects (roughly in chronological order), with a dedicated gallery of graphics that emerged from each of them. For more details check out my Publications page.
Project 1. Estimating internal wave turbulent dissipation in the deep ocean
My first physical oceanography paper (Lefauve, Melet & Muller, 2015) based on my MSc research tackled the turbulent dissipation of internal gravity waves generated by tides interacting with the rough seafloor in the deep, stably-stratified ocean (below the thermocline).
From the seafloor these waves propagate upwards, transporting their energy, and steepening as they encounter increasing stratification. This can cause them to break turbulently, dissipating kinetic energy, and mixing the surrounding waters in sometimes highly-localised regions. This mixing is important for the large-scale circulation of the oceans.
I produced three-dimensional worlwide maps of energy dissipation by combining linear and nonlinear theories (for the wave generation, propagation, and breaking) with three global datasets: small-scale bathymetry spectral data, tidal data (from satellite altimetry) and stratification data (from Argo floats).
I showed that more energy was dissipated near mid-ocean ridges in the Southern Hemisphere and that it was dissipated higher up in the water column than previously thought. I also proposed a simple method to include these results in global ocean models. This paper has been cited over 20 times by the fluid dynamics, oceanography, and climate change communities (in at least 12 different journals).
Project 2. Measuring turbulence in a canonical flow with state-of-the-art diagnostics
To better address the ocean mixing challenges revealed by Project 1, my PhD and postdoctoral research have then moved towards a more fundamental and experimental study of stratified turbulence.
Until recently, data-rich laboratory experiments were lacking because no diagnostics were capable of measuring three-dimensional, density-dependent, small-scale turbulence, and no laboratory flow could sustain for long time periods the high levels of dissipation found in Nature. I made such experiments possible by applying state-of-the-art diagnostics to a new, highly-dissipative, canonical stratified shear flow, the stratified inclined duct (abbreviated "SID").
The new methodology, originally developed in DAMTP by S. B. Dalziel and J. L. Partridge, uses three high-speed cameras and a fast-scanning laser sheet. It allows to measure the full three-component velocity and density fields in successive two-dimensional planes, which are then combined into volumes (Partridge, Lefauve & Dalziel, 2019).
I obtained 16 datasets of unprecedented quality which allowed me to develop the next two projects.
Today we are still developing these measurements in the G. K. Batchelor laboratory and keep pushing the boundaries with the latest laser and high-speed camera technology.
Project 3. Holmboe waves and three-dimensional coherent structures
Using the technology from Project 2, I uncovered experimentally and explained theoretically a new three-dimensional flow structure called a ‘confined Holmboe wave’ (Lefauve, Partridge, Zhou, Caulfield, Dalziel & Linden, 2018).
Holmboe waves are important interfacial oscillations found in oceans and estuaries, which transport energy and mass along and across fluid layers, and which can grow unstable and trigger more vigorous mixing. I explained mathematically how the classical Holmboe instability was modified in narrow rivers channels or deep-ocean trenches which severely confine the flow laterally. The agreement between a three-dimensional stability analysis and the experimental data was astounding. I gave seven talks on this work, created an artistic video for the APS DFD ‘Gallery of Fluid Motion’ and UK Fluid Network video competitions, and I received multiple awards and honours for it. The paper has already been cited over 20 times, including by three reviews and by four papers concerned with environmental applications.
This work spurred a MSc thesis project that I co-supervised with C. P. Caulfield and F. Gallaire (EPFL, Switzerland) to investigate more systematically the effects of confinement (Ducimetière, Gallaire, Lefauve & Caulfield, 2021). I then supervised a summer project on weakly nonlinear Holmboe waves to explain how these waves saturate to a finite (experimentally visible) amplitude. We developed and implemented numerically a challenging weakly nonlinear expansion to answer this question and provide a foundation for future fully-nonlinear work (Cudby & Lefauve, 2021). I also collaborated with the group of G. A. Lawrence (UBC, Canada) on fully-nonlinear simulations of Holmboe waves to higher amplitude, allowing for comparison with experiments. We focussed on the effects (or "feedback") that these waves have on the background flow through Reynolds stresses, which are central to modelling environmental flows (Yang, Tedford, Olsthoorn, Lefauve & Lawrence, 2021).
In a recent paper (Jiang, Lefauve, Dalziel & Linden, 2022) we looked at the evolution of coherent vortical structures under increasing turbulence intensity, using the 16 state-of-the-art 3D datasets from Project 2. We have been able to track the how the morphology of vortices and shearing structures evolve from confined Holmboe waves to give rise to characteristic hairpin vortices. This allowed to gain new insight into how vortices stir and mix the density field in previously unsuspected ways.